Enhanced water oxidation activity of Co3O4 ... - Wiley Online Library

DOI: 10.1002/celc.201600352

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Enhanced Water Oxidation Activity of the Cobalt(II,III) Oxide Electrocatalyst on an Earth-Abundant-MetalInterlayered Hybrid Porous Carbon Support Koshal Kishor+,[a] Sulay Saha+,[a] Sri Sivakumar,*[a, b, c, d] and Raj Ganesh S. Pala*[a, b] Supports over which electrocatalysts are deposited play a crucial role in the oxygen evolution reaction (OER), because they influence the surface roughness, morphology, electronic structure, and conductivity of electrocatalysts. In this context, we designed a hybrid carbon support having an earth-abundant metal as an interlayer between a Co3O4 electrocatalyst and a carbon support. The present approach resulted in an electrode that was three dimensional with high porosity, provided low resistance to parallel electron conduction pathways through the metallic interlayer, and modulated the electrocatalytic activity of Co3O4 by affecting its electronic structure. To rationalize the effect of the metal interlayer, a range of nonnoble earth-abundant metals (e.g. Cu, Mo, Ti, Al) were ex-

plored, of which the Cu-modified carbon support resulted in maximum specific activity (i.e. activity per electrochemical surface area) for the OER. Whereas both the electronegativity and conductivity of the metal interlayer were found to influence the OER activity, the dominant controlling factor in the explored systems was conductivity. The specific activity of the OER of electrodeposited Co3O4 was found to have a near-linear relationship between the electrical conductivity and the electron-donor metals (e.g. Ti, Al) and another near-linear relationship having different intercepts with electron-acceptor metals (e.g. Cu, Mo, W). The above understanding can be useful in increasing the OER activity of electrocatalysts through support– electrocatalyst interactions.

1. Introduction The strategic requirement for water electrolysis lies in finding a stable and inexpensive electrocatalyst that will facilitate a more hydrogen-driven economy.[1] A higher rate of the oxygen evolution reaction (OER) is obtained in doped RuO2 and IrO2 under acidic conditions;[2] however, these electrocatalysts are expensive. Inexpensive electrocatalysts can be obtained by combining earth-abundant transition metals (e.g. Ni, Co, Fe etc.), which are generally more stable in alkaline media and unstable in acidic media.[1, 3] These earth-abundant electrocatalysts show promising results, and their catalytic activity can [a] K. Kishor,+ S. Saha,+ Dr. S. Sivakumar, Dr. R. G. S. Pala Department of Chemical Engineering Indian Institute of Technology Kanpur, 208016 (India)

[b] Dr. S. Sivakumar, Dr. R. G. S. Pala Materials Science Programme Indian Institute of Technology Kanpur, 208016 (India) [c] Dr. S. Sivakumar Centre for Environmental Science and Engineering Indian Institute of Technology Kanpur, 208016 (India) E-mail: [email protected] [d] Dr. S. Sivakumar Centre for Nanoscience and Soft Nanotechnology Indian Institute of Technology Kanpur, 208016 (India) E-mail: [email protected] [+] These authors contributed equally to this work Supporting Information for this article can be found under: http://dx.doi.org/10.1002/celc.201600352.

ChemElectroChem 2016, 3, 1899 – 1907

be improved in the following ways: 1) by enhancing the surface roughness of the electrode, which can increase the number of electroactive sites with the formation of edges and steps and the exposure of high Miller-indexed surfaces;[2b, 4] 2) the use of highly porous three-dimensional electrodes.[2b, 3e, 4a, 5] Apart from this, the electrical conductivity and electronegativity of a support, on which the electrocatalyst is deposited, also play crucial roles in determining electrocatalytic activity. Further, a support can influence the morphology,[6] porosity,[3e, 7] and surface roughening of an electrocatalyst. An ideal support should, one, be stable under the reaction conditions; two, provide low charge-transfer resistance; three, lower the charge density around the metal atoms of the metal-oxide electrocatalyst through electrocatalyst–support interactions, which in turn lowers the activation barrier for the OER; four, provide mechanical stability to the electrocatalyst, which is often in the form of a nanostructure or thin film. Supports can be broadly classified into two groups: porous, threedimensional (3D) supports that can provide high surface area (e.g. graphene, Ni foam, silica, zeolite)[1b, 3a,e, 6–8] and smooth metal/metal-oxide supports that have high electrical conductivity[3a,b, 6, 8b–k, 9] (e.g. Ti, Au). The porous structures of carbon and zeolite supports enable a high electrochemical surface area (ECSA) to be achieved for the electrocatalysts, which results in higher activity, though they have lower electrical conductivities than metal supports, which thereby reduces their overall efficiency.[7, 10] Further, the high electronegativity of carbon promotes acidity in the catalyst sites.[9b, 8m] To alleviate the problem of low electrical conductivity, the use of metal supports has

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T 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Articles been suggested.[3e, 8b, 9e–g] In this regard, noble metals (e.g. Au, Ag, Pt, Ir) are an attractive choice due to their high electrical conductivities and electronegativities.[9f,g] Yeo and Bell showed an increase in the activity of OER electrocatalysts due to lowering of activation barrier for formation of CoIV in a Co3O4 thinfilm catalyst surface arising from Co–Au interactions.[9g] However, the prohibitive costs of these noble metals make them unattractive. To resolve these issues, some researchers have tried to use porous metallic frameworks such as Ni foam as support materials.[3e] In this regard, highly conductive metal interlayer frameworks made up of a range of non-noble earth-abundant metals (EAM = Cu, Mo, W, Sn, Ni, Zn, Ti, Fe, Al, Mn) supported on porous carbon were herein explored to increase the specific OER activity for electrodeposited Co3O4. The electrodeposition of a metal interlayer over a porous carbon support provides an additional electron percolation pathway, which leads to a significant increase in the specific activity and an insignificant reduction in the roughness of the porous support. The maximum OER activity is found for a Cu-modified metal support, and this high activity is attributed to its higher electrical conductivity; the OER activity of this electrocatalyst is about three times higher than that of the unmodified porous carbon support under similar conditions. This leads to a reduction in the potential by 0.15 V relative to the potential of the unmodified porous carbon support, and a current density of 20 mA cm@2 is achieved. Overall, we find that the specific activity of Co3O4 towards the OER is affected because of the following factors: 1) better electron percolation is achieved through the metallic interlayer, which is dependent on its conductivity; 2) the electronegativity of the support modifies the electronic structure of the electrocatalyst, but this effect is not substantial due to the thickness of the electrocatalyst ( & 10 monolayers); 3) the support induces electrocatalyst faceting. Specifically, the investigated metals can be broadly classified into two groups: electron donors (metals having lower electronegativity than Co; e.g. Zn, Ti, Al) and electron acceptors (metals having higher electronegativity than Co; e.g. Cu, W, Sn, Mo), and the latter class of metal interlayer provides higher activity for cases in which the electrical conductivities of the metals are similar. Furthermore, the OER activity of electrodeposited Co3O4 is found to have a near-linear relationship between the electrical conductivity and the electron-donor metals and a near-linear relationship with different intercepts for the electron-acceptor metals. Additionally, the electrical conductivity of the metal interlayer has a greater effect on the OER activity than the electronegativity of the metals for an approximately 10 ML thick electrocatalyst overlayer. The above understanding can be extended to enhance the OER activity of other electrocatalysts by using an inexpensive metal interlayer in a porous framework.


Experimental Section Materials Electrodeposition was performed on carbon paper (Thermax), and 0.1 m boric acid (99.8 % pure, Fischer Scientific) was used to maintain the pH during electrodeposition. Na2WO4·2 H2O (99.0 %), Na2MoO4·2 H2O (99.0 %), CuSO4·5 H2O (99.0 %), ZnSO4·7 H2O (99.1 %), CoCl2 (99.0 %), TiCl4 (99.0 %), NiSO4·6 H2O (98 %), Al2(SO4)3·H2O (98 %), MnSO4·H2O (96 %), FeCl3, and SnCl2 (99.8 %) were used as precursors for W, Mo, Cu, Zn, Co, Ti, Ni, Al, Mn, Fe, and Sn, respectively. All the precursor salts were purchased from Loba-Chemie. To increase the conductivity, 1.0 m NaCl (Fischer-Scientific) was used as a supporting electrolyte.

Synthesis of Co3O4 on a Metal-Interlayer-Modified Carbon Support Co3O4 electrocatalysts were synthesized by the potentiostatic electrodeposition method on a carbon support (electrode area: 1 cm2). Prior to electrochemical deposition, the carbon support was sonicated in acetone and then in methanol for 30 min to clean its surface. At first, a metal (EAM = W, Mo, Ni, Ti, Cu, Zn, Ti, Mn, Sn, Fe, Al) interlayer on the carbon support was prepared through electrodeposition in 1.0 m NaCl + metal precursor salt solution (for details, see Figures S1 and S2 in the Supporting Information). The electrodeposition of all the metal interlayers resulted in almost similar loadings ( & 0.4 mg) except for Sn. During preparation of the Ti metallic interlayer, a solution of 1 m Ti precursor salt + NaCl resulted in hydrolysis of the Ti salt in water and subsequent precipitation of hydroxide. To redissolve the precipitate in solution, it was further treated with 0.5 m H2SO4 with vigorous stirring for 1 h to yield a clear solution. Preparation of the Ti metallic interlayer on carbon paper was performed through electrodeposition of Ti by using the above-mentioned solution at an overpotential of @0.8 V for 20 min. The rate of electrodeposition of Sn was higher than that of the rest of the metals, which resulted in a higher metal loading. To ensure almost equal loading, Sn was electrodeposited on the carbon support chronoamperometrically in 1.0 m NaCl + metal precursor salt solution at an overpotential of @0.8 V for 2 min. The EAM-modified carbon support was dried under vacuum at room temperature for 12 h. On top of the EAM-modified carbon support, Co was electrodeposited by using 0.5 m CoCl2 + 1.0 m NaCl solution at a potential of @0.9 V for 20 min. All potentiostatic electrodepositions were performed at room temperature in a solution by using a three-electrode system assembly with Ag/AgCl as the reference electrode and Pt mesh as the counter electrode. After electrodeposition, the prepared anode was dried at room temperature for 2 h and then heated at 500 8C for 3 h in air.

Physiochemical Characterization XRD analysis were performed by using a Siemens D5000 Bragg– Brentano q–2 q diffractometer. Step-scan X-ray powder-diffraction data were collected over the 2 q range of 15–758 with Cu-Ka (40 kV, 40 mA) radiation with a diffractometer equipped with a diffracted-beam graphite monochromator crystal, 2 mm (18) divergence and antiscatter slits, 0.6 mm receiving slit, and incident beam Sollar slit. The scanning step size was 0.048 with a counting time of 1.5 s step@1. Surface morphology was determined by scanning electron micrographs by using field-emission scanning electron microscopy (FESEM, SUPRA 400VP Gemini, Zeiss). The surface elemental composition and chemical states of the components

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Articles were analyzed by XPS by using a PHI VersaProbe II Scanning XPS Microprobe. All XPS results were calibrated by setting the C 1s peak to a binding energy of 284.6 eV.

Electrochemical Characterization

1 1 ¼ þ kv 0:5 q qt

ð4Þ

The total voltammetric charge (qt) is calculated by extrapolating the linear plots at n = 0. The values of the outer charge (qo) can be calculated by extrapolating to v!1 in the plot of q versus [email protected] according to the following equation [Eq. (5)]:

Electrochemical characterization was performed by using a potentiostat (Autolab 302N, Metrohm India, Ltd.). The electrochemical activities of the synthesized electrocatalysts were analyzed in 0.5 m NaOH electrolyte solution with a scan rate of 50 mV s@1 by cyclic voltammetry (CV) measurements in an electrochemical cell with Pt mesh as the counter electrode and Ag/AgCl/(sat. KCl) as the reference electrode. Linear sweep voltammetry (LSV) measurements were performed at a scan rate of 5 mV s@1. CV and LSV of the electrodes were performed in a single-compartment cell by using a potentiostat. Electrochemical impedance spectroscopy (EIS) measurements were performed by using an Auto lab/FRA instrument in the frequency range of 1 kHz to 0.1 Hz with an ac perturbation of 10 mV. The impedance data was modeled by using Nova 1.9 software. All the potentials are reported with respect to Ag/AgCl if otherwise not mentioned.

q ¼ qo þ k1 v @0:5

To calculate the values of the ECSA of the prepared electrocatalysts, LSV was performed at different scan rates (1, 2, 5, 10, 20, 50, 100 mV s@1) within a potential range of 0 to 1.0 V versus Ag/AgCl. The current density was plotted against scan rate at a potential of 0.4 V versus Ag/AgCl. The double-layer charge capacitance measured at non-faradaic regime is a measure of the accessible ECSA.[11] This measurement involved calculating the slope of the obtained current density as a function of scan rate at a fixed potential [Eq. (1)]:

experimentally collected total volume of oxygen gas > 100 % theoretical amount of evolved oxygen gas ð7Þ

i ¼ cdl

dv dt

ð1Þ dv

in which i is the obtained current density and dt is the scan rate. The obtained value of Cdl (double-layer capacitance) can be correlated to the ECSA with following relation [Eq. (2)]:

ECSA ¼

C dl d er e0

ð2Þ

in which e0, er, and d are the electrical permittivity, relative electrical permittivity, and distance between the two layers, respectively (d = 10 a).[12] The roughness factor (RF) values of the electrocatalyst were calculated by using the following relation [Eq. (3)]:

RF ¼

ECSA GSA

ð3Þ

in which GSA is the geometrical surface area of the electrode (1 cm2 in the present case). The electrochemical porosity was calculated by using the procedure suggested by Trasatti and co-workers.[13] Accordingly, the total charge (qt) and outer charge (qo) of the metal oxide can be calculated by integrating the voltammograms at different scan rates. The voltammetric charge corresponding to the total surface area (qt) can be calculated by plotting the reciprocal of q against the square root of the potential scan rate (v) by using the following equation [Eq. (4)]: ChemElectroChem 2016, 3, 1899 – 1907

ð5Þ

and both k and k1 are constants of proportionality. The electrochemical porosity is measured as [Eq. (6)]:

Electrochemical porosity ¼

qt @ qo qt

ð6Þ

The evolved gas was collected and quantified by gas chromatography. The gas evolved from the anode corresponds to pure oxygen gas (Figure S4). The Faradaic efficiency is computed by using the following relation [Eq. (7)]:

Faradaic efficiency ¼

2. Result and Discussion 2.1. Evaluation of Activity on Different Supports for the OER The performances of the electrodeposited Co3O4 electrocatalysts over different supports were investigated through CV and LSV. The activities of the electrocatalysts were compared by examining the overpotential required to achieve a current density of 20 mA cm@2 and the current density obtained at an overpotential (h) of 0.4 V (Figure 1). Electrodeposited Co3O4 gave an activity of 6 mA cm@2 at h = 0.4 V on the carbon support due to the highly porous support framework. Though the obtained electrochemical porosity and roughness factor of Co3O4 were high (see Table 1), the activity/ECSA was low (1.48 mA cm@2) due to the high resistance of the electron pathway offered by the carbon support. To improve electron transport on the porous carbon support, an electrodeposited metal interlayer was added to provide an additional electron percolation pathway, and then electrodeposition of Co was performed over this hybrid support. To compare the OER activity of Co3O4 to that of the unmodified porous carbon support, the electrodeposition potential and duration for Co catalyst deposition were kept constant to ensure almost equal loading of the active Co3O4 electrocatalyst. Also, the loading of the metal interlayer was maintained at a constant level for each case and is given in Table 1. By using the Cu-modified carbon support, the current density underwent a threefold enhancement relative to that of the simple carbon support under the same reaction conditions, despite a small reduction in the porosity, and a decrease in the overpotential ( & 0.15 V) relative to that observed for the simple carbon support was also achieved. The as-prepared Co3O4 on Cu-interlayered carbon was stable at a potential of 0.6 V versus Ag/AgCl for 48 h and the activity de-



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Figure 1. a) Overpotential needed to reach 20 mA cm@2. b) Comparison of the current density of Co3O4 on different metal-interlayer-modified carbon supports at t = 0 and at t = 48 h at an overpotential of 0.4 V in 0.5 m NaOH. (c) LSV curves of the current density of the Co3O4 catalyst on the EAM-interlayer-modified carbon support in the potential range of 0 to 1.0 V versus Ag/AgCl in 0.5 m NaOH solution.

creased by about 10 %, but it showed a Faradaic efficiency of 92 % for the OER (Table S3). The enhancement in activity was attributed to the presence of the metallic interlayer. To demonstrate that the surface Cu atoms were not responsible for the enhanced OER activity, a comparison of the OER activity of co-electrodeposited CoCu oxide and the Co3O4 electrocatalyst on the Cu-metal interlayer carbon support was performed, wherein the activity as well as ChemElectroChem 2016, 3, 1899 – 1907

the specific activity were found to be much higher for the latter (Figure S3). Similar results were found on co-electrodeposition of Co-EAM (EAM = Cu, Mo, W, Sn, Zn, etc.); the activities towards water oxidation were lower than that on electrodeposited Co3O4, except in the case of Ni (Figure S3 a). This was the result of the formation of non-cobalt oxide surface clusters or surface terminations comprising metals other than Co (see Section 2.3. for additional proof), which are catalytically less active towards the OER than Co3O4 on co-electrodeposition, except in the case Ni. Notably, careful optimization of the synthetic conditions was undertaken to ensure minimal or no oxidation of the metallic interlayer (see Section 2.3). Oxidation of the metallic interlayer to semiconductor/insulator metal oxide resulted in a decrease in the OER activity, which was observed in control experiments performed specifically with Co3O4 on oxidized interlayers comprising Cu-CuO (Table S6). The RF and electrochemical porosity were calculated for electrodeposited Co3O4 through electrochemical means and are summarized in Table 1. The ECSA and RF for electrodeposited Co3O4 are lower than those of the individual carbon support, which suggests that electrodeposition of the metal interlayers leads to partial occlusion of the pores of carbon. The electrochemical porosity is lowered by 2.0 % with a moderate decrease in the RF due to Cu electrodeposition on the carbon support. The lowering of the electrochemical porosity is greater in the cases of the Mo-modified and W-modified carbon supports. However, this reduction in number of active sites and porosity has an insignificant effect on the overall current density for water oxidation, as the enhancement in electron transport through the support vitiates the effect of the reduction in porosity and number of active sites. The specific activity in the Cu-modified carbon support was almost three times higher than that of the unmodified carbon support. The water oxidation activities/ECSAs of the metal-modified carbon supports follow the order Cu > W > Mo > Sn > Al > Ni > Ti > Zn > Fe > Mn. The order of specific activity can be correlated to the order of conductivity of the support. To understand the mechanistic aspects of the electrocatalysts, LSV was performed. A lower value of the Tafel slope implies better activity of the electrocatalyst. The Tafel slope was around 47 mV dec@1 for the electrodeposited Co3O4 catalyst. The Tafel slope indicates that deprotonation of @OH is the rate-determining step for the OER (S@OH!S@O + H + + e@) as per the reaction mechanism proposed by Bockris.[14] The Tafel slope of Co3O4 on the modified carbon support indicates a small reduction in the value but no change in the reaction mechanism with regard to the OER. At higher overpotentials, hydration of the surface becomes the rate-determining step (S + H2O!S@OHads + H + + e@). Further, to understand the electron transport along the electrode–electrolyte interface, electrochemical impedance spectroscopy (EIS) was performed at a working electrode potential of 0.7 V (vs. Ag/AgCl) (h = 0.5 V). At this overpotential, the surface layer of Co is irreversibly transformed into the CoIV state or CoO2 species, and under these conditions, surface hydration (S + H2O!S@OHads + H + + e@) is the rate-determining step, as the OER follows an Eley–Rideal reaction mechanism.[15] The far-



Articles Table 1. Comparison of the activities of electrodeposited Co3O4 on different EAM-modified carbon supports.

EAM

Conductivity of metallic interlayer [MS m@1]

Electronegativity of metallic interlayer (Pauling Scale)

Current density per ECSA [mA cm@2] at an overpotential of 0.4 V

RF (electrochemical porosity [%])

Charge transfer resistance per ECSA [mW]

Loading of metallic interlayer [mg]

Cu Al W Mo Zn Ni Fe Sn Ti Mn unmodified carbon support

59 38 20 20 17 14 10 9.1 2.5 0.62 0.1[a]

1.90 1.61 2.36 2.16 1.65 1.91 1.83 1.96 1.54 1.55 2.54[a]

4.38 3.16 3.95 3.56 2.48 3.01 2.34 3.45 2.63 1.91 1.48

5797 1772 4781 5090 2939 3025 2093 3812 2108 2356 6292

0.13 0.51 0.14 0.17 0.34 0.30 0.49 0.24 0.49 0.64 0.34

38.2 41.2 41.3 38.8 38.1 39.0 36.1 41.5 41.2 42.0 –

(78.74) (54.71) (65.6) (69.28) (62.25) (64.07) (53.1) (63.68) (56.33) (57.7) (80.21)

[a] Corresponds to the electronegativity and electrical conductivity of carbon in the graphite phase.

adaic impedance (Ytotal) under these conditions is given by [Eq. (8)]: 1 Rct ¼ Rs þ Y total 1 þ jwRct C dl

ð8Þ

which corresponds to a Rs(RctQdl) circuit (Rs = solution resistance, Rct = charge-transfer resistance, Qdl = constant phase element representing surface heterogeneity along the electrode– pffiffiffiffiffiffiffiffi ffi electrolyte interface, j = imaginary number ( @1Þ, w = 1 rad s@1). The above-mentioned circuit suggests a single semicircle in the Nyquist plot (Figures S5 and S6), which was found from the experimentally obtained data and fits well with the simulated data. However, Rct represents the total resistance along the electrode–electrolyte interface and does not provide information regarding the activation barriers of the individual active sites for the OER. To qualitatively evaluate Rct per active site, we divided the obtained value of Rct by the roughness factor. Among the different metal-modified carbon support explored, the charge resistance for water oxidation for electrodeposited Co3O4 follows the order Cu & W < Mo < Sn < Ni < Zn < Ti & Fe < Al < Mn. The decrease in the charge-transfer resistance is due to the formation of the metal interlayer within the porous carbon support; this provides an additional electron percolation pathway, which thereby facilitates water oxidation at the electrocatalyst–electrolyte interface. The higher the conductivity of this additional electron percolation pathway, the lower the charge-transfer resistance at the electrocatalyst–electrolyte interface will be. For this reason, the Cu-modified carbon support provides a minimum charge-transfer resistance, whereas the carbon support provides a maximum chargetransfer resistance.

2.2. Morphological Investigation through SEM The surface morphology of electrodeposited Co3O4 and the individual supports was investigated through scanning electron microscopy (SEM). The obtained images are shown in Figure 2. Figure 2 a shows the irregular morphology of the electrodeposChemElectroChem 2016, 3, 1899 – 1907

Figure 2. SEM images of electrodeposited Co3O4 on different supports: a) Cu-modified carbon support and b) carbon support. SEM images of c) carbon support and d) Cu-modified carbon support.

ited Co3O4 particles distributed over the fibrous carbon support, which suggests that the porous and fibrous characteristics of the carbon support are able to provide more sites for electrodeposition. The SEM images of electrodeposited Co3O4 on the modified carbon support (Figure 2 b) show a porous network, but it is smaller than the uncoated carbon support (Figure 2 a). The reason for this loss in porosity was attributed to deposition of the metal layer over the carbon support, and this can be inferred by comparing the SEM image of the simple carbon support (Figure 2 c) to that of the metal-modified carbon support (Figure 2 d). Because Co3O4 is a relatively stable electrocatalyst, long exposure to an OER environment does not result in a visible change in the morphology, as observed by SEM (Figures S9–S11). Also, the morphology of Co3O4 varies upon changing the metallic interlayer due to support-induced faceting (Figures S9–S11).



Articles 2.3. Chemical State and Phase of the Electrocatalyst on Different Supports The chemical state and phase of the electrocatalyst is important in determining its activity. XRD analysis shows that electrodeposited Co is transformed into Co3O4 on heat treatment (Figure 3). Both the crystallinity and relative peak intensities of Co3O4 change with the support (Figure S7). The crystallinity of Co3O4 is higher on the Cu-modified carbon support than on the unmodified carbon support. The peak intensity of the (220) plane of Co3O4 is considerably lower than that of the (311) plane, and the (222) peak is almost invisible if Co3O4 electrodeposited on the carbon support. However, upon formation of Co3O4 on the Cu-modified carbon support, the peaks of the as-mentioned planes become considerably enhanced, and the (220) peak is the most intense. This happens due to the conformal growth patterns of the different planes that vary with

Figure 3. XRD patterns of electrodeposited Co3O4 on Cu-modified carbon support, electrodeposited Co3O4 on carbon support, and carbon support.

the supports. Notably, both in-plane and out-of-plane orientations can be controlled by choosing the support and the deposition parameters, as observed for other electrodeposited oxide films.[6, 16] The activities of the various planes of Co3O4 are different due to dissimilar packing densities and electronic structures.[17] This leads to variations in reactivity, depending on the ratio of exposed surfaces,[18] as the reactivity is dependent on the exposed surfaces as well as the potential.[17] Though thermal and electrochemical oxidation of electrodeposited Co leads to the formation of Co3O4, it does not lead to oxidation of the Cu interlayer, as observed by XRD (Figure 3). The Co 2p XPS shows peaks at binding energies of about 780–782, 786, and 796–797 eV, which correspond to the Co 2p3/2, Co 2p satellite, and Co 2p1/2 peaks, respectively (Figure 4 a).[18, 19] Notably, it is difficult to deconvolute CoII from CoIII in the Co 2p spectra, as the peak positions differ by only 0.4 eV.[9d, 20] The spin–orbit level energy spacing of the Co 2p3/2 and Co 2p1/2 peaks is 15 eV, which is similar to Co3O4 found in the literature.[9d, 19b,c] The existence of a satellite peak at a binding energy of around 786 eV is attributed to the presence of paramagnetic CoII species, which have a d7 configuration.[20, 21] The presence of a more electronegative metal interlayer as a support shifts the Co 2p3/2 peak towards higher binding energies due to Co–EAM interactions. Given that Cu, Mo, and W are more electronegative than Co, they act as electron sinks, which facilitates the oxidation of CoIII to a higher oxidation state (Co3 + + EAM!Co3 + d + EAMd@). The Pauling electronegativity order of the metals is W > Mo > Cu, and accordingly, the Co 2p3/2 peak is found at binding energies of 781.34, 781.12, and 780.64 eV, respectively. On the other hand, EAMs such as Ti are less electronegative than Co, and they act as electron sources, which promotes the reduction of CoIII to a lower oxidation state (Co3 + + EAM!Co3@d + EAMd + ). The Co 2p3/2 peak is also shifted toward a lower binding energy (780.6 eV), as observed in Figure 4 a. Further, from the O 1s spectra it was possible to differentiate the extent of crystal oxygen, hydroxylated oxygen, and struc-

Figure 4. XPS spectra of a) Co 2p in the 778–800 eV zone at the Cu-, W-, Mo-, and Ti-modified carbon supports and b) O 1s in the 526–544 eV zone at the Cu-, W-, Mo-, Ti-, and Al-modified carbon supports.




Articles tural water in the electrodeposited Co catalyst. Three types of peak are found in the O 1s spectra, that is, nonhydrogenated O bonded to Co atoms at a binding energy of 530.9 eV, OH bonded to Co atoms at a binding energy of 532.4 eV, and the O atoms of structural water at a binding energy of 535.9 eV (Figure 4 b), and these correspond to the peaks assigned in the literature.[19c] On comparing the ratio of the first two peaks, we concluded that the extent of hydration in the unmodified carbon support was greater than that in the modified carbon support. Moreover, the extent of hydration differed depending upon the metal interlayer used in making the modified carbon support. The extent of non-hydrogenated O was found to be the highest in the case of the Cu-modified support. Electrodeposition on the W support led to a lower degree of hydroxylation than that found in the Mo-modified carbon support. It was also found that the extent of oxidation was higher for modified supports for which the metal interlayer had a higher electronegativity than Co. The presence of a larger oxidized surface in the modified carbon support increases the rate of the OER. The surface activity of electrodeposited Co3O4 was attributed to the transition of CoII !CoIII !CoIV during water oxidation.[3b, 9g] The presence of a relatively electronegative metal as a support would help to increase the population of CoIV at the surface by reducing the energy barrier for CoIII !CoIV oxidation. The effect of the support acting as an electron sink was demonstrated by comparing the activity of Co electrodeposited on a W-modified carbon support to that of Co electrodeposited on a Mo-modified carbon support. Mo and W have similar electrical conductivities, but W is more electronegative, and hence, it can act as a better electron sink than Mo, as exemplified by the higher activity as well as the higher specific activity obtained in the case of the W-modified carbon support. The feature of the electron sink was also exemplified by the higher degree of non-hydroxylated oxygen: the hydroxylated oxygen ratio in the Co3O4 electrocatalyst of the W-modified carbon support is higher than that in the Co3O4 electrocatalyst of the Mo-modified carbon support. Notably, the effect of electronegativity has a major ramification in deciding the electrocatalytic activity only if the electrocatalyst thin film is not more than 3–5 monolayers (ML), as pointed out previously by Yeo and Bell.[9g] However, the 3D electrocatalysts producing higher activities are far thicker than about 3–5 ML. The deposited electrocatalysts of the present study have thicknesses of around 10 ML (Table S2), but their activities are still influenced by the electronegativity of the support. Also, if a less electronegative metal is used as the metal interlayer, the metallic interlayer is prone to oxidation. The Ti interlayer in the Ti-modified carbon support was found to undergo oxidation, as evidenced from its XPS spectra (Figure S13 b), in which the three peaks of Ti 2p are found at binding energies of 455, 459.2, and 462.2 eV. The first and last peaks are assigned to the Ti 2p3/2 and Ti 2p1/2 states of metallic Ti, whereas the peak at a binding energy of 459.2 eV is assigned to TiIV 2p3/2.[2b] The formation of the metal oxide lowers the electronic conductivity of the support, which thereby defeats the purpose of the metal interlayer. However, no such oxide formaChemElectroChem 2016, 3, 1899 – 1907

tion was found if a highly electronegative metallic interlayer was used, as exemplified by the metallic states found in the Cu-, W-, and Mo-modified carbon supports. In the XPS spectra of the Cu-, W-, and Mo-modified carbon supports, peaks at binding energies of 932.1 eV for Cu 2p, 31.3 eV for W-4f7/2, and 232 and 235.8 eV for Mo 3d signifies that the metals were not converted into oxides (Figure S13).[19a] The compositions on the different layers of co-electrodeposited CuCoOx and electrodeposited Co3O4 on the Cu-interlayered carbon support were investigated by sputtering and XPS. It was found that Cu was present even in the surface layer for the co-electrodeposited electrocatalyst, the but presence of Cu was at a greater depth in the electrodeposited Co3O4 on the Cu-interlayered carbon support electrocatalyst, as evident from Figure 5(top). This suggested a decrease in the activity of the co-electrodeposited electrocatalyst due to the formation of Cu clusters at the surface. Similar results were obtained for coelectrodeposited NiCoOx and electrodeposited Co3O4 on the Ni-interlayered carbon support (Figure S14).

Figure 5. Top) Depth profile study of Co-electrodeposited CuCoOx electrocatalyst and electrodeposited Co3O4 on Cu-interlayered carbon support. Bottom) XPS of the Co3O4 electrocatalyst on the Cu-modified carbon support before and after electrochemical water oxidation.

The metal interlayers are not oxidized under the electrochemical OER conditions, as they are protected by the electrocatalyst overlayer, except in the case of Ti. After 48 h of chronoamperometry at 0.6 V versus Ag/AgCl, XPS was again performed on the catalyst to examine the oxidation state of the Cu interlayer of the Co3O4 electrocatalyst deposited on the Cuinterlayered carbon support [Figure 5(bottom)]. No visible significant shifts in the XPS peaks of Cu 2p3/2 and Cu 2p1/2 were observed before and after water oxidation. These twin peaks indicate that Cu was present as a metallic interlayer, which in-



Articles dicates that the metal interlayer was stable under the electrochemical OER conditions. 2.4. Discussion As mentioned earlier, the specific activity of Co3O4 on different metallic interlayers depends on electrical conductivity, the electronegativity of the metallic interlayer, and the exposed facets of the electrocatalyst. However, the electronegativity of the metal interlayer does not play a major role in determining the activity of the Co3O4 electrocatalyst if the electrocatalyst is more than 5 ML thick.[9g] This is demonstrated by the fact that the Cu-modified carbon support provides higher OER activity than the W- and Mo-modified carbon supports, despite the fact that the electronegativity of Cu is lower than that of either W or Mo (Figure S16). To correlate the effect of the electrical conductivity of the metallic interlayer on the activity of the Co3O4 electrocatalyst, the electrical conductivity of the interlayer metal support was plotted against the specific activity at an overpotential of h = 0.4 V (Figure 6). A dominant correlation between electrochemical specific activity and electrical conductivity was observed.

Figure 6. Plot of the activity/ECSA at an overpotential of 0.4 V (vs. Ag/AgCl) versus conductivity of Co3O4 on the metallic interlayer of different EAMmodified carbon supports.

The plot reveals two classes of metals: 1) metals that are more electronegative than Co; 2) metals that are less electronegative than Co. The conductivity of the interlayer-forming metal has an almost-linear relationship with the specific activity of the Co3O4 electrocatalyst. Similarly, interlayer-forming metals that are less electronegative than Co also have a near-linear relationship between conductivity and the specific activity of the overlayer Co3O4 electrocatalyst, even though the near-linearity may not have any specific mechanistic significance. In fact, as the electrochemical specific activity is a convolution of three effects, that is, conductivity, electronegativity, and dissimilarity in electrocatalytic activities of compositionally similar active sites, a linear correlation was not expected. In addition to a support being a better conductor, if it is also a better electron acceptor (more electronegative than Co) and ChemElectroChem 2016, 3, 1899 – 1907

exposes more active electrocatalytic surface sites, it is anticipated that the specific activity of Co3O4 would be “supralinear” to the linear correlation attempted in Figure 6. If the electrical conductivities of the metals are comparable, then the electronegativity of the interlayer-forming metal would be the decisive factor, even for an electrocatalyst that is about 10 ML thick. This is demonstrated by the W-modified and Mo-modified carbon supports, which have similar electrical conductivities, but due to the higher electronegativity of W, the specific activity of the overlayer-forming Co3O4 electrocatalyst has higher OER activity than the latter. However, if the support is a poor electron acceptor (less electronegative than Co) and exposes fewer electroactive sites, such data points may show “sublinear” behavior. This is particularly observable for the Mninterlayered carbon support, upon which the Co3O4 electrocatalyst shows the lowest specific activity among the considered metals. Lastly, both the crystallinity and exposed facets of the overlayer electrocatalyst would also depend on the metallic interlayer, which was demonstrated by the various morphologies obtained as seen by changes in the relative intensities of the peaks in the XRD pattern as the metallic interlayer was varied. Support-induced electrocatalyst faceting also leads to variations in the specific activity and stability,[17, 18] and this makes all compositionally similar sites on the catalyst surface electrocatalytically dissimilar. Overall, it is to be noted that if we attempt to modulate the electrocatalytic activities by metallic interlayers, proper care should be taken to retain the metallic state of the metal interlayer while treating it under either electrochemically or thermally oxidative conditions. The major probable problem in these types of metal interlayers is that they may get oxidized and form either semiconductor or insulator metal oxides, which would lead to a decrease in the OER activity of Co3O4 due to a decrease in conductivity.

3. Conclusions In the present work, the problem of low electrical conductivity associated with porous carbon supports was circumvented by depositing a metal interlayer and then electrodepositing an OER electrocatalyst over it. This resulted in marginal decreases in the porosity and surface roughness but a significant increase in the specific activity towards the OER. We attributed the increase mainly to the higher electrical conductivity provided by the metal-modified support, variation of the exposed surfaces of Co3O4, and tuning of the oxidation characteristic of the catalyst overlayer through electrocatalyst–support interactions. Maximum OER activity was found for the Cu-modified metal support, which was ascribed to its higher electrical conductivity; its OER activity was about three times higher than that of the unmodified porous carbon support under similar reaction conditions, and furthermore, the overpotential was reduced by 0.15 V relative to that of the unmodified porous carbon support. The specific OER activity of the Co3O4–metal-interlayered electrocatalyst increased almost linearly with the conductivity of the support. In addition to being a better conductor, if the



Articles support was a better electron acceptor (more electronegative than Co) and exposed more active electrocatalytic surface sites, the specific OER activity of Co3O4 increased “supralinearly” with the conductivity of support. In contrast, if the support had poor electron-acceptor properties (less electronegative than Co) and exposed fewer electroactive sites, it showed “sublinear” behavior of specific OER activity of Co3O4 vis-/-vis conductivity of the support. We hope such correlations will serve as qualitative heuristics to facilitate the design of materials for the OER.

Acknowledgements We thank the Indian Space Research Organization (ISRO) (Grant No. STC/CHE/20110043) and the Technology Systems Development Program of the Department of Science and Technology (Grant No. DST/TSG/SH/2011/106) for supporting this work. We also thank Dr. S. Illangovan and Mr. A. Senthil Kumar, Vikram Sarabhai Space Centre, ISRO, and Dr. S. Ravichandran, Central Electrochemical Research Institute, Karaikudi, for useful discussions during the course of this work. Special thanks to Ashok Kumar Umireddy for helping with gas chromatography. Keywords: cobalt oxide · metallic interlayers · oxygen evolution reaction · porous hybrid carbon supports · threedimensional electrodes [1] a) J. R. Gal#n-Mascarjs, ChemElectroChem 2015, 2, 37 – 50; b) S. Jung, C. C. L. McCrory, I. M. Ferrer, J. C. Peters, T. F. Jaramillo, J. Mater. Chem. A 2016, 4, 3068 – 3076. [2] a) T. Audichon, S. Morisset, T. W. Napporn, K. B. Kokoh, C. Comminges, C. Morais, ChemElectroChem 2015, 2, 1128 – 1137; b) K. Kishor, S. Saha, M. K. Gupta, A. Bajpai, M. Chatterjee, S. Sivakumar, R. G. S. Pala, ChemElectroChem 2015, 2, 1839 – 1846. [3] a) X. Deng, W. N. Schmidt, H. Tuysuz, Chem. Mater. 2014, 26, 6127 – 6134; b) J. B. Gerken, J. G. McAlpin, J. Y. C. Chen, M. L. Rigsby, W. H. Casey, R. D. Britt, S. S. Stahl, J. Am. Chem. Soc. 2011, 133, 14431 – 14442; c) I. Abidat, N. Bouchenafa-Saib, A. Habrioux, C. Comminges, C. Canaff, J. Rousseau, T. W. Napporn, D. Dambournet, O. Borkiewicz, K. B. Kokoh, J. Mater. Chem. A 2015, 3, 17433 – 17444; d) X. Deng, H. Tuysuz, ACS Catal. 2014, 4, 3701 – 3714; e) X. Lu, C. Zhao, Nat. Commun. 2015, 6, 1 – 7. [4] a) X. Zhou, Z. Xia, Z. Tian, Y. Ma, Y. Qu, J. Mater. Chem. A 2015, 3, 8107 – 8114; b) X. Yu, Z. Sun, Z. Yan, B. Xiang, X. Liu, P. Du, J. Mater. Chem. A 2014, 2, 20823 – 20831. [5] S. Chen, J. Duan, P. Bian, Y. Tang, R. Zheng, S. Z. Qiao, Adv. Energy Mater. 2015, 5, 1500936. [6] J. A. Koza, Z. He, A. S. Miller, J. A. Switzer, Chem. Mater. 2012, 24, 3567 – 3573. [7] M. Zhang, Y.-L. Huang, J.-W. Wang, T.-B. Lu, J. Mater. Chem. A 2016, 4, 1819 – 1827. [8] a) T. Sharifi, E. Gracia-Espino, X. Jia, R. Sandstrçm, T. W,gberg, ACS Appl. Mater. Interfaces 2015, 7, 28148 – 28155; b) S. Yusuf, F. Jiao, ACS Catal. 2012, 2, 2753 – 2760; c) S. Chen, Y. Zhao, B. Sun, Z. Ao, X. Xie, Y. Wei, G. Wang, ACS Appl. Mater. Interfaces 2015, 7, 3306 – 3313; d) C. C. L. McCrory, S. Jung, J. C. Peters, T. F. Jaramillo, J. Am. Chem. Soc. 2013, 135, 16977 – 16987; e) A. Minguzzi, F.-R. F. Fan, A. Vertova, S. Rondinini, A. J.


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Manuscript received: June 23, 2016 Accepted Article published: August 4, 2016 Final Article published: August 30, 2016

